1. Field of the Invention
The present invention relates to a light source of a non-self-luminous display device, and more particularly to a liquid crystal display device having a backlight which includes a light guide plate and uses an LED as a light emitting element.
2. Background Art
Recently, the liquid crystal display device has been popularly used as a display device. Particularly, the liquid crystal display device is used as a display part of portable equipment because of a reduced thickness, a reduced weight and small electric power consumption.
However, the liquid crystal display device is not self-luminous and hence, the display device requires a lighting means. In general, as a lighting device which is used for the liquid crystal display device, a flat lighting device referred to as a backlight is popularly used. Conventionally, although a cold cathode fluorescent tube has been used as a light emitting element (also referred to as a light source) of the backlight, an LED (light emitting diode) has been also recently used as the light emitting element.
The sidelight-type backlight includes a plate-like light guide plate. The light guide plate is made of a light-transmissive resin material or the like. The light emitting element is arranged on a side surface of the light guide plate. Light which is incident on the light guide plate from the light emitting element propagates in the inside of the light guide plate. Reflection and diffusion members such as grooves, projections or printed materials are formed in or on the light guide plate, and the light which propagates in the inside of the light guide plate is radiated toward a liquid-crystal-display-device side by the reflection and diffusion members.
In the light guide plate used in the display device, slight difference in quantity of light lowers quality of the display device. Accordingly, it is preferable to make quantity of light radiated from the light guide plate uniform over the whole area of a light radiation surface of the light guide plate.
JP-A-2004-192909 proposes a technique which changes an apex angle of prisms formed on a light radiation surface of a light guide plate corresponding to a distance from a light emitting element for making a quantity of light radiated from the light guide plate uniform.